1. Field of the Invention
The present invention relates to a power transmission shaft to be used as, for example, a drive shaft or a propeller shaft, which constitutes a part of a power transmission system in an automobile.
2. Description of the Related Art
In general, there are several kinds of transmission shafts that constitute a power transmission system of an automobile. The shafts include a drive shaft for connecting between an engine and a wheel-bearing device, a propeller shaft for transmitting power from a transmission to reduction gears, and so on. Each of these shafts has a coupling member such as a spline on the shaft-end. The power transmission shafts may be broadly classified in the group of solid shafts made of solid bars and the other group of hollow shafts made of steel pipes or the like, according to their basic structures.
Conventionally, solid shafts have been used as power transmission shafts for automobiles. In recent years, for responding to the needs for higher function of automobiles, the sound insulating properties of a cabin to keep quiet, and the like, there are increasing demands of providing a power transmission shaft with various kinds of characteristic features, such as light weight, compactness, and comfortability against NVH (noise, vibration, and harshness), in addition to strength and durability. In addition, there is also required to improve the torsional rigidity of shafts for increasing the controllability and direct feeling of automobile at the time of start. In this case, for improving the torsional rigidity, there is an idea of increasing the diameter of the shaft. However, it will effect an increase in costs because of increasing the weight and the cutting amount of a coupling portion. In addition to the above demands, there is a need for adjusting the natural frequency of automobile for avoiding the noise produced by a resonance between vibrations of an engine and a shaft while the automobile runs. For adjusting the natural frequency, there is an idea of attaching a dumper or the like on the power transmission shaft. However, it will lead to an increase in costs because of increasing the number of structural components and the number of assembling steps in the manufacturing process.
As a consequence of considering the above demands in terms of functions, there is an increasing tendency to make greater use of hollow shafts instead of the solid shafts. The hollow shafts can be broadly divided into integral-type and joined-type. The integral-type hollow shaft comprises a middle pipe part having the largest outer diameter and shaft parts integrally formed on the opposite ends of the pipe part. The shafts parts are made of the same material as that of the middle pipe part and a coupling portion such as a spline is formed on the outer periphery of each shaft-end. On the other hand, the joined-type hollow shaft comprises a pipe part and shaft parts. These parts are shaped separately and are then joined together using friction pressure welding, electric welding, or the like.
Comparing with the solid shaft, the integral-type or joined-type hollow shaft has a reduced section modulus, while the maximum shear-stress thereof operative to the hollow shaft is large. Therefore, there is a possibility of a decrease in the shear strength of the hollow shaft.
In some cases, an electro-resistance-welded tube having a wall thickness with an extremely high accuracy and an extremely stable strength is used as a power transmission hollow shaft. The welded tube is comprised of two or more pipe parts. The pipe parts are made of a steel material having a good dimensional accuracy and a good finishing accuracy and are butt-joined in a straight line using electric resistance welding. Therefore, the welded portion of an electro-unite part of the welded pipe, which extends in the axial direction, tends to be broken, leading to a decrease in the strength of the power transmission shaft.
In addition, the integral-type hollow shaft for power transmission is typically formed by, for example, a swaging in which the diameter of an element tube is reduced by stamping in the radial direction thereof at high speed, while rotating the tube around the axis; or a press working in which the diameter of an element tube is reduced by inserting the element tube into a die. The hollow shaft formed by such a plastic working of the swaging or the like may have a plastic flow of the raw material into the inner radial at the time of reducing the diameter of the element tube. Thus, there is a tendency in which the inner radial surface of the hollow shaft become wrinkled. Such a wrinkle may become the origin of breakage, causing a decrease in the strength of the power transmission shaft.